1. Field of the Invention
The present invention relates to an objective for compact lens-shutter cameras whose diaphragm is arranged behind the whole optical system, and more particularly to a compact wide angle objective with a simple construction for cameras for a spread use.
2. Description of the Prior Art
With a compact lens-shutter camera for cameras for a spread use, a wide angle objective with a diaphragm arranged behind the whole lens system is advantageous for accommodation in the camera, for compactization of the camera or the objective itself and for realization of precise focusing. As this kind of objective, triplet type objectives with F.sub.No. of 4-2.8 and a total angular field coverage of 60.degree.-64.degree. have been proposed. With a wide angle triplet type objective, since an astigmatic difference tends to become large at a marginal angular field, it was conventional to construct this type of objective by the use of lens elements formed of a glass with a high refractive index. For example, in Japanese patent application laid-open No. Sho. 59-34510, all lens elements, one or two of which have an aspherical surface, are formed of glass with a high refractive index to construct an objective with a total angular field coverage of 64.degree., and F.sub.No. of 3.5. In Japanese patent application laid-open No. Sho. 57-2012, an objective with F.sub.No. of 4 and a total angular field of 64.degree. has been realized with the use of a glass with a high refractive index. But forming a lens element of glass of a high refractive index incurs an increase in the manufacturing cost.
Besides, with a triplet type objective with a diaphragm arranged behind the whole lens system, a problem exists in that the amount of marginal light rays tends to become insufficient and this promblem has not been solved.